Speed regulator



July 5 1932- H. M. sToLLER ET L 1,366,275

SPEED REGULATQR y (Jxiginal4 Filed June 23, 1927 5 Sheets-Sheet l@mae/f:

BYMW

July 5, 1932.

H. M. sToLLER ET AL. 1,866,275

SPEED REGULATOR original Filed June 23, 1927 a sheets-sheet 2 July 5,1932. H. M.l sToLLER 'ET Al.

SPEED REGULATOR Original Filed June 25. 1927 3 Sheets-Sheet 5 HIIIHHHH/WENm/gfgggggo ATTORNEY Patented July 5, 1932 UNITED STATES PATENTOFFICE f HUGH M. STOLLER, OF MOUNTAIN LAKE, NEW JERSEY, AND EDMUND R.MORTON, OF

BROOKLYN, NEW YORK, ASSIGNORS TO BELL TELEPHONE LABORATORIES, INCOR-PORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK SPEED REGULATOROriginal application led June 23, 1927, Serial No. 200,799, and in JapanApril 5, 1928. Divided and this application led March 28,

sired phase relation.

This invention is applicable, for example, to television systems inwhich the maintenance of close synchronization' and a desired phaserelation of the moving scanning elements of the transmitter and receiveris required- Such moving elements may be driven by direct current motorsin which case means must be provided for maintaining v these motors insynchronous operation.

'In accordance with this invention a high frequency alternating currentgenerator is mechanically coupled to the motor at the controllingstation and the current therefrom is supplied to a similar machinecoupled to the direct current motor at the controlled station and isthereby operated as a synchronous motor. The stator of the highfrequency machine at the receiving station is provided with a Worm gearrotated by a hand operated worm for permitting framing of the receivedimage. This arrangement is operable but it has two disadvantages. First,it is difficult to synchronize the machines as it is diiiicult to closethe armature circuit of the high frequency motor at the right phaseposition with the motors running at very closely the same speed, and,second` the stator of the high frequency machine at the receivingstation may have to be turned through a large anglewhen framing theimage, which is diflicult to accomplish as the movement can not be maderapidly without throwing the motor out of step.

.ln accordance with a further aspect of the invention` thesedisadvantages are overcome by the provision of a low frequency interlockin addition to the high frequency interlock above described. Thelow'frequency interlock provides an approximate synchronization within asingle limited range of phase positions thereby greatly 'facilitatingthe processes of synchronizing on the high frequency and of framing.

The invention is illustrated in the accom- 1930. Serial No. 439,683.

panying drawings in which Figs. 1, 2 and 3, placed end to end, show atelevision system employing the circuit arrangement of the presentinvention for synchronously operating the driving motors, and Fig. 4shows the arrangement for mounting the motors at the receiving station.

This invention as illustrated in the drawings comprises a televisionsystem in which the scanning discs at the transmitting and receivingstations are driven by two-pole direct current motors individualthereto. The speed ofthe motor at one station, which will hereinafter bedesignated as the control station, is maintained constant by a speedregulating arrangement. The direct current motor at the control stationis mechanically coupled to a high frequency generator of the inductortype and a low frequency wave is obtained from slip rings connected toopposite com mutator bars of the motor. This low frequen- /cy Wave andthe high frequency Wave from the synchronous generator are used forsynchronizing the operation of the second direct current motor, that is,the one at the controlled station.v In order to facilitate thetransmission of the low frequency Wave, it is caused to modulate bymeans of a. relay the output of an electro-mechanical oscillator andthis modulated wave, together with the high frequency Wave generated bythe synchronous generator, are transmitted over the same line to thecontrolled station where after being amplified they are separated bywave {ilters. The modulated wave is passed through a dcinodulator, theoutput current of which operates a polarized relay causing its contactsto open and close at the frequency generated at the slip rings of themotor at the control station. The low frequency square Wave from thecontacts of this relay is supplied to the grid circuit of athree-electrode electric discharge detector or control device, the spacecurrent of which is supplied from slip rings connected to oppositecommutator bars of the motor at the controlled station and the resultingoutput current, the value of which isa function of the relative phase ofthe low frequency Waves supplied to the grid and plate circuits, issupplied to a regulating field winding of the .as explained above.

controlled direct current motor. In starting this motor, ,the action ofthese low frequency waves through the detector circuit is to pull thetwo motors into step at the desired phase position and to hold themthere within limits of about 20 degrees. After this the high frequencywave from the synchronous generator is supplied through an amplifier toadynamo electric machine of the inductor type mechanically coupled to thecontrolled direct current motor. This dynamo electric machine may act asa motor, aiding the direct current motor, or as a generator, opposingthe direct current motor, in which case the power generated isdissipated as heat in the machine and amplifier tubes. This highfrequency interlock maintains a precise phase position be tween the twodriving motors at some one of a number of points within which they wereheld by the low frequency interlock. The scanning discs may then beadjusted to the relative phase positions required for proper framing ofthe images by the use of mechanical means which rotates the statorAstructures of the machines at the receiving station. After the twomotors are brought into synchro nism and the image framed, it has beenfound that usually the most satisfactory operation is obtained by usingonly the high frequency interlock. As is obvious to one skilled in theart, the framing may also be accomplished by the use of suitablemechanical means for rotating the scanning dise on the shaft, or bycausineF the high frequencymachine to slip one pole at a time afterdisconnecting the low frequency interlock. In order to further increasethe accuracy of synchronization, the rotating system at the controlledstation is designed to have a moment of inertia small compared to thatof control station.

In some cases it may be desirable to transmit the low frequency wavedirectly from one station to the other, but usually where theinterconnecting lines are of any considerable length it is preferable toemploy that Wave for modulating a wave of higher frequency, Obviously,if desired, the two synchronizing waves may be transmitted over separatelines. Obviously, for short lines of negligible impedance andsubstantial power capacity the machines may be directly connectedwithout the use of devices for synchronizing with the small amounts ofpower which it is possible to transmit over telephone lines.

The use of both high and low frequency synchronization is desirable forthe reason that it is diiiicult to synchronize on the high frequencyalone, since the motors must be brought to the same speed within acertain limit expressed in rate of change of phase position inelectrical degrees. For machines of a large number of poles this will bea very minute difference in speed of rotation. By synchronizing first onthe low frequency and mits the synchronization of the machine rati, asmany phase ositions as there are pairs of poles. This eature of thesystem is par-W ticularly advantageous in systems which require that themotors can be interlocked at one-definitephase position, for example, as

vis required to give accurate framing of the picture in a scribed.

In some systems, particularly where the motor` at the controlled stationdisplays a marked tendency to vvary in speed, due to television systemas herein de- `power supply voltage fluctuations or transmissiondisturbances such as are to be ex. jpected With radio transmission, thecontinuous use of the double frequency synchro- `nization may be foundadvantageous. This is desirable if there is 'probability of the highfrequency control being overcome, 1n kwhich case'the machines would haveto be synchronized by the operator if the low frequency interlock werenot in use. If, on the contrary, it were in use, the machine wouldlimmediately resynchronize, with only a Iriomentary disturbance of thetransmitted picture.

Referring to the drawings, there are shown two stations Aand B whichcomprise a television system of the type disclosed and claimed in thecopending application of F. Gray, Serial No. 181,538, filed April 6,1927. At the station B, the subject 101 whose image is to be viewed atthe receiving station A by the receiving operator 102 is located infront of a scanning apparatus. This apparatus comprises an arc lamp 103and an optical system for causing a slender pencil of substantiallyparallel rays of intense light to sweepacross the field in front of thescanning apparatus and scan the subject 101 in a series of successiveparallel lines. The optical system includes a scanning disc 104 whichhas a number of small circular apertures 106 arranged in a spiralextending around the disc near its rim. -Light from the arc 103 isdirected by a lens system 107 so that an intense beam of light passesthrough each aperture as it moves across the illuminated area of thedisc. An opaque screen 108 in front of the disc is provided with arectangular opening 109 of such width as to admit light from only oneaperture at a time. Lens 110 bends the slender pencil of parallel rayspassing through the aperture and the opening 109 to form an image of themovin aperture on the subject 101. As a result o th1s arrangement thesubject is completely scanned once for each revolution of the disc in aseries of type of scanning apparatus is described in more detail in theapplication of Gray referred to above.

The image waves generated in the photoelectric cell are amplified in anelectric discharge amplifier 113 and transmitted through a transformer114 to a line 115 connecting the transmitting station B with therecelving station A.

The scanning disc 104 is driven by a direct current motor 20 connectedthereto throu h the shaft 80 and gear box 81. Obviously, t e gear boz'rmay be omitted and the disc 104 mounted directly upon the shaft 80 ifdesired. The motor 20 is arranged to be driven in synchronism with acontrol motor located at the receiving station A. The relative loca- 80tion of the control and controlled motors is,

in general, immaterial, the arrangement shown herein being preferablewhere a receiving system is arran ed to be connected with any one 'of anum r of transmitting $5 stations. The operation of the synchronizingcircuit is hereinafter explained in detail. At the receiving station Athe image Waves from the line 115 are transmitted through a transformer116 to an electric discharge am- 4o plifier 117 in which theyare'amplified and fed to a neon glow discharge lamp 118 ar ranged infront of a scanning disc 121 similar in construction to the scanningdisc located at the transmitting station B. The disc is driven by thedirect current motor 10 connected thereto through the shaft 90 and thegear box 91. In front of the disc there is located an opaque screen 119having an opening 120 of such size that at one time only one of theapertures of the disc 121 is in the field of view of the observer 102.The lamp 118 is preferably of the type disclosed in detail in thecopending application of F. Gray, Serial No. 138,831, filed October 1,1926. The optical field defined by the opening 120 and illuminated bythe light coming from the lamp 118 through the moving apertures of thedisc 121 is viewed by the observer 102 Without the aid of any opticalsystem. 00 The observer sees at any instant a single aperture inthe samerelative position as the spot of light on the subject 101 at thetransmitting station B and the brightness ofthe aperture corresponds tothe amount of light a5 refiected from that particular elemental area ofthe subject. On account of persistency of vision, the observerconsequently sees an apparent image of the subject on the front surfaceof the disc 121, complete views of the Subj ect being transmitted at therate of about 18 per second so that the subject canbe seen in motion.

In order to provide for proper framing of the image the direct currentmotor 10 and the alternating current generator 15 are mounted in asuitable cradle base 160 as shown in Fig. 4. The mounting is so arrangedthat the stator structures of both may be simultaneously rotated througha portion of a revolution to adjust the phase position of the scanningdisc to obtain proper framing of the image. This is accomplished bymeans of the worm gear 161 which is attached to the stator structure ofboth units and which engages the worm 162 mounted in suitable bearingsattached to the base 160. By turning the worm 162 in one direction orother by means of the hand Wheel 163 the angular position of the statorstructures of the machines 10 and 15 may be simultaneously'changed, thuscausing their armatures and consequently the scanning dise to take thephase position required for proper framing.

The circuit for synchronously operating the driving motors 10 and 20will nov:7 be described. Each of these motors is a direct current motorof the compound wound type having, respectively, series field windings11 and 21, shunt field windings 12 and 22 and regulating field windings13 and 23. All of the windings of each motor are cumulative. Each of themotors is provided with a pair of slip rings, 14 and 24, respectively,connected to opposite commutator bars for producing an alternatingcurrent of a low frequency of one cycle per revolution of the motor.Alternating current generators of the inductor type 15 and 25 aremechanically coupled to the motors 10 and 20, respectively. Thegenerators comprise, respectively, rotors 16 and 26 and stators 17 and27 on which are mounted exciting windings 18 and 28 and generatingwindings 19 and 29. The generator 25 operates as a synchronous motor, ashereinafter described.

The speed of the .motor 10 is maintained constant by a regulatingcircuit of the type described in the patent to H. M. Stoller, No.1,662,085, issued March 13, 1928. This regulating circuit comprises atuned circuit 30 consisting of a condenser 31 and an inductance coil 32connected in series. This circuit is connected to the generating Winding19 and the terminals of the inductance coil 32 are connected in theanode circuit of a threeelectrode electric discharge rectifier ordetector V33 for supplying space current thereto, the voltage suppliedto the anode circuit being proportional to the drop across theinductance coil 32. Connected in series in the anode circuit of thedetector 33 is a resistance 34 shunted by condenser 9. This resistanceis also connected in the input circuits of the three-electrode electricdischarge devices 35 and 36, so that the grid voltage of these devicesis controlled by the potential drop across it. The regulating fieldwinding 13 is connected in the anode circuit of the devices 35 and 36 inseries with the source 38. The power supply for the cathodes of thedetector 33 and the devices 35 and 36 and for the anode circuits of thedevices 35 and-36 is obtained from the transformer 37, the primarywinding of which is connected to the slip rings 14. The source of power38 supplies direct current to the motor and also serves as a source ofpositive grid biasing potential for the devices 35 and 36, theconnection being made in series with resistance 34. A condenser 39 isconnected in shunt to the regulating field winding 13 to steady therectified current supplied thereto from the anode circircuits of thedevices 35 and 36. Two resistances 40 and 41 are connected in seriesacross the regulating field winding 13. The junction of these tworesistances is connected to the grid of the detector 33 through a thirdresistance 42 and a condenser 43 is connected in shunt to the seriescircuit comprising resistances 40 and 42.

The motor is started by closing the switches 67 and after it is broughtup to speed the switch 44 is closed, energizing the detector 33 and thedevices 35 and 36 so that the normal speed is held constant by theaction of the regulating circuit. The tuned circuit is so designed thatthe frequency of the wave generated in the winding 19 when the motor isrunning at the desired speed lies on the falling portion of itsresonance curve. When the speed of the motor starts to increase, forexample, the voltage drop across the inductance 32 decreases, reducingthe space current of the detector 33 and consequently the lating fieldcurrent voltage drop across the resistance 34 which opposes the positivebiasing potential impressed on the grids of the devices and 36 from thesource 38, thus increasing the reguand limiting the rise in the motorspeed. The precision of the circuit is increased by the delayedfeed-back to the grid of the detector 33. Any increase in the platecurrent of the devices 35 and 36 increases the voltage drop across theresistance which is applied through the resistance 42 to the grid of thedetector 33. However, due to the action of the condenser 43 and theresistance 42, this voltage does not increase immediately but lagsbehind by a time which is proportional to the value of the resistance 42and the capacity of the condenser 43. This time should preferably be ofthe same order or greater than the natural period of oscillation of thesystem. The operation of the circuit to prevent any decrease in thespeed of the motor is ana ogous.

The motor 20 is maintained in synchronism with the motor 10 by means ofthe high frequency wave generated in the winding 19 and the lowfrequency wave obtained from the slip rings 14. These Wavescould .betransmitted over separate lines to the television transmitting stationB, but in the circuit herein shown they are arranged to be transmittedover the same line. As the wave generated at' the slip rings 14 is oflow frequency (of the order of 17 c. p. s.) it is undesirable totransmit it directly .over a telephone line. The output of a higherfrequency wave is therefore modulated by this low frequency wave and theresultant modulated wave transmitted to the station B.

This higher frequency wave, which was, as used, of the order of 760cycles per second, for example, is generated by an oscillator 130 whichis preferably of the electromechanical type, as described in thecopending application of C. R. Moore, Serial No. 48,223, filed August5,1925. The output of the oscillator is connected through the transformer'131, the secondary winding of which is shunted by a high resistance 132,to the input of the low pass filter 133. The terminals of the secondarywindings of the transformer 131 are connected to the contact andarmature of the relay 134, the Winding of which is.

connected through the leads 46 to the slip rings 14 so that the outputof the oscillator is short circuited at the frequency of the wavegenerated at the slip rings 14, thus modulating the 7 60 cycle wave fromthe oscillator. This modulated wave is transmitted through the low passfilter 133 and the transformer 135 to the line 136 which connects thestations A and B. The generator winding 19 is connected through theleads 45 to the input terminals of the high pass filter 137 the outputterminals of which are connected to the transformer 135 so that the highfrequency wave is also transmitted over the line 136 to the station B.

At the station B the high frequency wave and the modulated wave aretransmitted through the transformer 138 to the electric dischargeamplier 139. Connected in parallel to the output of the amplifier 139are the high pass filter 140 and the low pass filter 141. The high passfilter 140 is designed to select the high frequency wave generated inthe winding 19, and the low pass filter 141 is designed to select themodulated wave which is then transmitted through the transformer 142 tothe electric discharge demodulator 143.

This demodulator is provided with the usual cathode heating, gridbiasing and space current batteries. Connected to the output cf thedemodulator 143 are the windings of a polarized relay 144, the armatureof which is operated by the demodulated current at the frequency of thewave generated at the slip rings v14. The operation of the relayproduces a square-top wave of interrupted current from the battery 145.This wave and the high frequency wave in the output of the filter' 140are employed for operating the motor 20 in synchronisin with the motorl0.

Thesynchronizing circuitcomprises a pushpull control or detector circuitD employing two three-electrode discharge devices 47 and 48, and apush-pull amplifier Afm, employing two three-electrode discharge devices49 and 50. The cathodes of the four devices are supplied with heatingcurrent from the 110 volt line 3l-52. llhis line also supplies gridbiasl ing potential to the devices 49 and 5U through the resistances 54and 55. bpace current for the devices 49 and 50 is supplied from the 750 volt line Lfil- 58. bpace current is supplied to the devices 47 and48 from the 110 volt line l-Z and the transformer 58, the primarywinding of which is connected to the slip rings 24. The low frequencywave from the relay 144 is supplied to the input circuit of thedetectors 47 and 48. The regulator .lield winding 28 is connected in theoutput circuit of the detector l) to receive recti- Iied currenttherefrom, and condenser 59 is provided for by-passing the alternatingcomponent of this current. Condenser 60 is for the purpose of powerfactor correction, neutralizing the lagging current drawn by transformer8 which 1s built to draw an unusually heavy magnetizing current. Thistransformer condenser combination acts to prevent hunting of -theregulated motor system. The value of the current supplied to theregulating iield 23 depends upon the relative phase of the low frequencywaves supplied to the input and output circuits of the detector l) sothat this portion of the synchronizing circuit operates to maintain thespeed of the motor such that the wave from the slip rings 24 is in anapproximately constant' phase relation with the corresponding ,lowfrequency wave from the relay 144 in a many ner similar to the operationof the regulating circuit described in the Patent No. 1,663,890 to H. M.Stoller, March 27, 1928. The high pass filter 140 is connected to theamplifier Am through the leads 150 and transformer 151 so' that the highfrequency is again amplified and supplied through the output transformer61 to the winding 28, driving the machine as a synchronous motor, thusholding the motor 20 substantially exactly in phase with the motor 10.condenser 62 is connected in series with the secondary winding of thetransformer 61 and the winding 28 to tune the circuitV to a frequency alittle higher than the synchronous frequency of the ma chine 25. vThistends to prevent hunting as of the high frequency waves. After themachinos are thus brought into synchronism the phase position of thescanning disc 121 can be ad] usted to the position required for properframing of the image by turning the hand wheel 168. l1`he low frequencyinterlock may then be disconnected and the high frequency interlockemployed alone for maintaining synchronous operation.

However, as explained above, it may be desirable in some cases tocontinuously employ both interloclrs in order to maintain mostsatisfactory operation.

ln one system in which the frequencies of the low and high frequencywaves were respectively 17.7 and 2125 cycles per second, it was foundpossible to maintain phase positions of the motor within 0.05o underordinary variations of voltage and load.

For best operation it is desirable that the rotating system associatedwith the control motor 10 should have a large moment of inertia so as toaid in holding the speed oonstant while the moment of inertia ofrotating system associated with the controlled motor 20 should berelatively smaller so that the ioo motor will readily follow any changesinA 4 the speed of the motor 10. rlhis is accomplished in the systemherein shown by locating the control motor 10 at the receiving station Aat which the large scanning disc 121 is used and locating the controlledmotor 20 at the transmitting station where the smaller scanning disc 104islused.

This application is a division of application Serial No. 200,799, filedJune 28, 1927, which in turn is a continuation in part of applicationSerial No. 181,314, filed April 6, 1927.

What is claimed is:

1. In combination, two driving motors, means for .deriving a wave of lowfrequency from and proportional to the speed of one of said motors,means operated by said one motor for generating a wave of high frequencyproportional to the speed of said one motor,

means for deriving a wave of low frequency.y

from and proportional to the speed of the other of said motors, a highfrequency synchronous motor mechanically connected -to said other motor,means controlled by the phase difference of said low frequency waves forregulating the speed of said other motor,

and connections for supplying said high frequenqy wave to saidsynchronous motor.

2. combination according to the preceding claim in which the means forregulating the speed of said other driving motor comprises an electricaldischarge detector having mput and output circuits, and connections forsupplying the low frequency waves to said input and output circuits,respectively,

10 to obtain an output current proportional to the phase difference ofsaid low frequency Waves. f

3. A combination according to claim 1 in which the low frequency wavehas a frequency of one cycle per revolution of said one driving motor.

4. In combination, a direct current motor having a commutator, sliprings connected to opposite bars of said commutator for obtaining a Waveof a frequency proportionalto the speed of said motor, a high frequencygenerator mechanically coupled to said motor, means for regulating thespeed of said motor, a second direct current motorhaving a commutator,slip rings connected to opposite bars of said commutator for obtaining aWave of a frequency proportional to the speed of said second motor, anelectric discharge detector having input and output circuits, means forsupplying waves from the slip rings of said first and second motors tosaid input and output circuits, respectively, a regulating eld windingfor said second motor connected in the output circuit of said detector,a high frequency synchronous motor mechanically coupled to said seconddirect current motor, and means for supplying Waves from said generatorto said synchronous motor.

5. The method of regulating the speed of a moving element whichcomprises utilizing one simple Wave of low frequency for roughlycontrolling the instantaneous speed ofsaid element, utilizing a secondsimple wave of high frequency to effect a iinedriving control of theinstantaneous speed of said element, and discontinuing the use of saidwave of low frequency.

6. The method of synchronously operating two driving elements whichcomprises transmitting a wave of relativelyt low frequency proportionalto the speed of one of said elements to the other element, utilizingsaid wave in controlling the operation of the other of said elements tobring said elements approximately into a desired phase relation,transmitting a wave of comparatively high frequency proportional to thespeed of said first element to the other element, utilizing said highfrequency Wave in controlling the operation of said second element toeffect a high degree of sychronous operation, and disconltinuing the useof said low frequency contro 7. The method of operating a motor insynchronism with a second motor which comprises utilizing the phaserelation of a Wave of low frequency proportional to the speed of saidsecond motor and a second low frequency wave of frequency proportionalto the speed ofsaid rst motor in controlling the Speed of said firstmotor to bring said first motor. approximately into a desired phaserelation with respect to said second motor, utilizing a wave of highfrequency proportional to the speed of said second motor in controllingthe speed of said rst motor to maintain substantially exact synchronismand discontinuing the use of said low frequency control.

8. The method of regulating the speed and phase of one moving elementwith respect to another which comprises utilizing one simple Wave of lowfrequency for bringing one of .the moving elements approximately into adesired phase relation with respect to the other, utilizing a secondsimple Wave of relatively high frequency for maintaining the movingelement at one of a plurality of positions within a range determined bysaid low frequency wave, and discontinuing the use of Said low frequencycontrol.

9. In combination, a rotatable element at a station, means located awayfrom said station for generating a simple Wave of low frequency and asimple Wave of high frequency, adirect current motor for rotating saidelement, means for modulating a higher frequency wave with said lowfrequency wave and for transmitting said modulated Wave and the highfrequency wave over one circuit to said station, means at said stationfor demodulating said modulated wave, means responsive to said lowfrequency wave and means at said station responsive to said highfrequency wave, said two responsive means serving jointly to control thedriving of said rotatable element.

l0. A combination, according to claim 9 in which said means responsiveto a low frequency wave is associated with said direct current motor.

1l. A combination in accordance with claim 9 in which said meansresponsive to a low frequency Wave controls a driving torque greaterthan that supplied by said means responsive to said high frequency wave.

12. In combination, a plurality of rotatable elements, a plurality ofdirect current motors one of which is associated with each of saidrotatable elements for driving it approximately in synchronism withanother of said rotatable elements, a high frequency motor mechanicallycoupled to each of said direct current motors for maintaining saidrotatable elements more accurately in synchronism, and means responsiveto a low frequency simple wave associated with each`of said directcurrent motors for bringing said rotatable elements approximately into adesired phase relation with respect to each other.

13. In combination, a plurality ofmovable elements separated by a`considerable distance, a motor for driving each of said movableelements,y a source of waves having a. frequency of one cycle perrevolution of one of said movable elements, a source of carrier waves,means for modulating the carrier Waves from said source in accordancewith Waves from said first mentioned source of Waves, means fordemodulating said modulated carrier waves, a line connecting saidmodulating and demodulating means, means controlled by said demodulatedwave for pulling one of said motors approximately into a desired phaserelation with respect to the other motor, a source of waves of highfrequency proportional to the speed of said second motor, a synchronousmotor mechanically coupled to said first motor, and means including atleast a portion' of the line connecting said modulating and demodnlatingmeans for impressing said high frequency Wave on said synchronous motorto maintain said first motor in substantially exact synchronism withsaid second motor.

In Witness whereof, We hereunto'subscrbed our names this 13th day ofMarch, 1930.

HUGH M. STOLLER. EDMUND R. MORTON.

